The present invention generally relates to limited rotation motor systems, and relates in particular to systems and methods for designing and adjusting limited rotation motor systems.
Limited rotation motors generally include stepper motors and constant velocity motors. Certain stepper motors are well suited for applications requiring high speed and high duty cycle sawtooth scanning at large scan angles. For example, U.S. Pat. No. 6,275,319 discloses an optical scanning device for raster scanning applications.
Limited rotation motors for certain applications, however, require the rotor to move between two positions with a precise and constant velocity rather than by stepping and settling in a sawtooth fashion. Such applications require that the time needed to reach the constant velocity be as short as possible and that the amount of error in the achieved velocity be as small as possible. Constant velocity motors generally provide a higher torque constant and typically include a rotor and drive circuitry for causing the rotor to rotate about a central axis, as well as a position transducer, e.g., a tachometer or a position sensor, and a feedback circuit coupled to the transducer that permits the rotor to be driven by the drive circuitry responsive to an input signal and a feedback signal. For example, U.S. Pat. No. 5,424,632 discloses a conventional two-pole limited rotation motor.
A limited rotation torque motor may be modeled or represented by a double-integrator model plus several flexible modes and low frequency non-linear effects. A typical closed-loop servo system for a galvanometer includes integral actions for low frequency uncertainties and a notch filter for high frequency resonant modes. System operation is chosen at the mid-frequency range where the system is well modeled by the rigid body. For a double integrator rigid body model, there is a direct relationship between the open-loop gain and the cross-over frequency on the frequency response plot. For example, an automatic timing system for a servowriter head positioning system is disclosed in Autotuning of a servowriter head positioning system with minimum positioning error, Y. H. Huang, S. Weerasooriya and T. S. Low, J. Applied Physics, v. 79 pp. 5674-5676 (1996).
FIG. 1 shows a limited rotation torque motor system 10 of the prior art. The system 10 includes a controller 12 (e.g., a position, integral, derivative or PID controller) that receives an input command 14. The controller 12 provides a control signal to a motor 16, which moves an optical element such as a mirror to provide position changes 18 responsive to the input command 14. The system also includes a position detector 20 that provides a position detection signal 22 that is also provided to the controller 12 with the input command 14. Open-loop gain (or 0 dB cross-over variations) of the system affects closed-loop system performance if the controller is not adaptive to these variations.
Such limited rotation motors may be used, for example, in a variety of laser scanning applications, such as high speed surface metrology. Further laser processing applications include laser welding (for example high speed spot welding), surface treatment, cutting, drilling, marking, trimming, laser repair, rapid prototyping, forming microstructures, or forming dense arrays of nanostructures on various materials.
Limited rotation torque motors, however, eventually fail after finite usage. As methodologies are developed to drive limited rotation motors harder, failure may come at unanticipated times. The ability to gauge the condition of a limited rotation motor is helpful in predicting the life of a motor. Moreover, it is desirable to be able to gauge the condition of a limited rotation motor in situ without requiring that the motor be return to the manufacturer for analysis.
There is a need, therefore, for a method of monitoring a limited rotation torque motor, and more particularly, there is a need for a method of diagnosing the health and life of a limited rotation torque motor.